Update Apt configuration to no longer explicitly prefer *.gz

[tianon]

This was added back when Acquire::GzipIndexes only applied to literally gzip-compressed sources -- it's since been generalized (https://salsa.debian.org/apt-team/apt/commit/b0f4b486e6850c5f98520ccf19da71d0ed748ae4; src:apt version 1.0.9.2, jessie and newer), so this is only necessary on older Apt versions.

Thanks to @lamby for prompting me to take another look at this section of the configuration tweaks we apply (more comments/opinions are always welcome, especially around these contents -- we've tried to include fairly verbose comments explaining the rationale behind each change). ❤️

[lamby]

Pleasure :)

[tianon]

The last thing we want is the current DPL (and a master of reproducibility whose work we've shamelessly borrowed from) thinking we're doing really weird things that maybe we could explain better (or even do better)! 😅 ❤️

Relevant diffoscope for wheezy:

@@ -1,11 +1,14 @@
 # Since Docker users using "RUN apt-get update && apt-get install -y ..." in
 # their Dockerfiles don't go delete the lists files afterwards, we want them to
-# be as small as possible on-disk, so we explicitly request "gz" versions and
-# tell Apt to keep them gzipped on-disk.
+# be as small as possible on-disk, so we explicitly request that Apt keep them
+# compressed on-disk too instead of decompressing them.
 
 # For comparison, an "apt-get update" layer without this on a pristine
 # "debian:wheezy" base image was "29.88 MB", where with this it was only
 # "8.273 MB".
 
 Acquire::GzipIndexes "true";
+
+# https://salsa.debian.org/apt-team/apt/commit/b0f4b486e6850c5f98520ccf19da71d0ed748ae4; released in src:apt 1.0.9.2, 2014-10-02
+# prior to src:apt 1.0.9.2, "Acquire::GzipIndexes" _only_ applied to gzip-compressed list files, so we need to prefer those on older releases
 Acquire::CompressionTypes::Order:: "gz";

The same, but for stretch:

@@ -1,11 +1,10 @@
 # Since Docker users using "RUN apt-get update && apt-get install -y ..." in
 # their Dockerfiles don't go delete the lists files afterwards, we want them to
-# be as small as possible on-disk, so we explicitly request "gz" versions and
-# tell Apt to keep them gzipped on-disk.
+# be as small as possible on-disk, so we explicitly request that Apt keep them
+# compressed on-disk too instead of decompressing them.
 
 # For comparison, an "apt-get update" layer without this on a pristine
 # "debian:wheezy" base image was "29.88 MB", where with this it was only
 # "8.273 MB".
 
 Acquire::GzipIndexes "true";
-Acquire::CompressionTypes::Order:: "gz";

[lamby]

\o/ diffoscope!

[tianon]

Aha, this has caused #41 in Jessie, so we'll need to bump up that minimum APT version to Stretch's instead (since Stretch is just fine). Very odd symptoms there (APT trying to download a package nobody asked for), took embarrassingly long to figure out this was the cause. 😅

What's really odd is that it works if I go back in time to 2018-01-01. 🤷‍♂️